In some electronic circuit applications there is a need to charge an electric charge storage element, like a capacitor, until a voltage across the charge storage element reaches a given threshold voltage. A charging circuit for charging the charge storage element can include a depletion MOS transistor which has its drain-source path connected in series with the charge storage element and which has its gate terminal connected to a terminal for a reference potential, wherein the series circuit with the depletion MOS transistor and the charge storage element is connected between supply voltage terminals. When in such an arrangement the charge storage element is uncharged, i.e. when the voltage across the charge storage element is zero, and when a supply voltage is applied between the supply terminals, a charging process starts which results in an increase of the voltage across the charge storage element. When the voltage across the charge storage element increases, the gate-source voltage of the depletion MOS transistor changes, wherein the depletion MOS transistor switches off (pinches off) when the gate-source voltage reaches its pinch-off voltage. In such applications the reference potential of the gate electrode and the pinch-off voltage of the transistor are selected or adjusted such that the transistor pinches off when a desired voltage across the charge storage element is reached.
A depletion MOS transistor includes a channel region and a gate electrode which are dielectrically insulated from one another by a gate dielectric. The channel region is n-doped in an n-type transistor and is p-doped in a p-type transistor. In an n-type transistor the majority charge carriers are electrons and the minority charge carriers are holes, while in a p-type transistor the majority charge carriers are holes and the minority charge carriers are electrons. In both of these types of depletion transistors, minority charge carriers can accumulate in the channel region. These minority charge carriers are, for example, generated by thermal charge carrier generation. Minority charge carriers accumulated in the channel region can influence the pinch-off voltage of a depletion transistor. In a worst scenario the minority charge carriers may prevent the depletion transistor from switching off at all.
There is, therefore, a need for a depletion transistor which reliably switches on and off dependent on its set pinch-off voltage.